stupid question first - are you certain that there's nothing preventing the knob/tap from screwing all the way in?
in terms of a diy fix, do you actually use the hot water? if you are technically minded you could either remove and re-build the valve or simply plug the pipe. the latter is probably not ideal (nor as straight forward as it sounds - a simple fix like a bolt in the boiler outlet may give rise to some other interesting problems).
the former is not so complicated, but i'm not entirely sure how user serviceable the valves are. my BFC steam valve is intermittently sealing shut which tells me it needs a rebuild with some food grade lubricant but i haven't gotten around to it yet so i'm not really of any help. if you need a new valve you may need to talk to a supplier directly as i've not seen diadema/bfc specific items for sale online.
usual disclaimers apply, there's 240vac and hot water under pressure in that box so be careful